1. Field of the Invention
The present invention relates to a photoelectric converting device consisting of static induction transistors or punching-through bipolar transistors of a novel lateral junction structure. Such photoelectric converting device is particularly adapted for use in a solid-state image sensor.
2. Related Background Art
The photoelectric converting device utilizing static induction transistor (SIT) is already known in a vertical structure and a lateral junction structure.
The SIT of lateral junction structure has the advantages of (1) easier control and less fluctuation of device characteristics (particularly current-voltage characteristics) among different SIT's, and (2) possibility of size reduction, because of the smaller diffusion depth of the accumulating gate, in comparison with the vertical structure.
A conventional lateral-junction SIT is shown in FIGS.2A and 2B, which are respectively a schematic plan view and a schematic perspective view.
On an n-substrate 21 there is formed a p.sup.+ -embedded gate region 25, on which formed is an n.sup.- -channel region 22. In the surface layer of said, region 22 there are formed an n.sup.+ -source region 23, an n.sup.+ -drain region 24 and a p.sup.+ -surface gate region 26 positioned therebetween. The gate regions 25, 26 are bonded by plural grating gate regions 27.
A drain current flows in n.sup.- -channel regions 22 of widths w, said w being the distance of the grating gate regions 27, and is controlled by a gate potential varying according to the amount of photocharge accumulated in the gate, regions 25, 26, 27.
A separating region 28 separates the pixels, each composed of a photoelectric converting device.
On the p.sup.+ -surface gate, region 26, there is formed a gate electrode 30 across an oxide film 29, thus constituting a MOS capacitor C.sub.G 31. This is required for resetting and other purposes, but may be dispensed with in case of a single pixel. The same applies to the separating region.
The conventional lateral-junction SIT has been subjected to following drawbacks:
(1) A complex manufacturing process is required for the formation of the p.sup.+ -embedded gate region 25 and the grating gate, regions 27;
(2) The characteristics of the SIT are adversely influenced by the presence of fluctuations in the gap w of the grating gate regions 27. Since the grating gate regions 27 have to be diffused deep until the embedded gate region 25, the diffusion is difficult to control and the gap w of gratings inevitably shows fluctuation. Consequently, the characteristics (particularly current-voltage characteristics) of the photoelectric converting devices often fluctuate and are unstable;
(3) Making the photoelectric converting device compact is difficult, due to the necessity of the deep embedded gate region 25 and the deep diffused grating gate regions 27.